Bicharge zinc oxide

ABSTRACT

Conventional photoconductive zinc oxide is converted into bicharge photoconductive zinc oxide by heating with selenium in air at about 300*-600*C. Bicharge zinc oxide is particularly useful as the phtoconductive pigment in photocopy papers.

United States Patent [191 Bowman [451 Apr. 2, 1974 BICHARGE ZINC OXIDE[75] Inventor: Robert S. Bowman, Pittsburgh, Pa.

[73] Assignee: St. Joe Minerals Corporation, New

York, NY.

[22] Filed: Jan. 29, 1973 [21] Appl. No.: 327,670

Related U.S. Application Data [63] Continuation-in-part of Ser. No.188,520, Oct. 12,

1971, abandoned.

[52] U.S. Cl 96/l.5, 96/l.8, 252/501 [51 Int. Cl. G03g 5/00, 603g 7/00[58] Field of Search 96/1.8, 1.5; 252/501 [56] References Cited UNITEDSTATES PATENTS 3,060,134 10/1962 Elder et al. 96/l.8 X

Brandon 96/1 .8 X

Primary Examiner-Norman G. Torchin Assistant Examiner-John L. GoodrowAttorney, Agent, or FirmHarold L. Stowell [57] ABSTRACT Conventionalphotoconductive zinc oxide is converted into bicharge photoconductivezinc oxide by heating with selenium in air at about 300600C. Bichargezinc oxide is particularly useful as the phtoconductive pigment inphotocopy papers.

6 Claims, No Drawings BICHARGE ZINC OXIDE This application is acontinuation-in-part of application Ser. No. 188,520, filed Oct. 12,1971 now abandoned.

This invention relates to the production of bicharge photoconductivezinc oxide.

Photocopy papers are generally prepared by coating a system containingphotoconductive zinc oxide on a paper base. Conventional (monocharge)photoconductive zinc oxides perform well in negative chargephotocopying. Bicharge zinc oxide, however, can accept and dischargeboth negative and positive charges and can be used in the production ofeither negative or positive photocopy. Such dual capability adds muchflexibility to the practical use of photocopy papers coated with thephotoconductive zinc oxide and is particularly desirable in microfilmcopying.

No commercially acceptable bicharge zinc oxide is available at present.While materials alleged to have bicharge properties have been announced,they are offcolored and do not perform well in the positive charge mode.Positive photocopies made using such materials are very low in qualityand have an objectionable tan color.

I have discovered a simple, direct method for convetting conventionalphotoconductive zinc oxide into a white (i.e., colorless), bichargephotoconductive material which, when coated on photoprint paper,provides good quality photocopy in both the negative and the positivemodes. According to my method, selenium as the metal or in an oxidizedform, is physically admixed with the zinc oxide and the admixturecontaining 0.02 to 2.0 weight percent of selenium based on zinc oxideinitially present is heated in air at 300600C. The residence time attemperature is selected to vaporize the selenium uniformly throughoutthe mass of'zinc oxide. in a preferred embodiment zinc oxide and 0.1-1.0weight percent of powdered selenium metal 'are heated in static air at400-500C for about -30 minutes. The resultant bicharge product issomewhat whiter than the starting zinc oxide, and is free flowing,odorless, and possesses an oxidized surface. A similar product isobtained by heating zinc oxide in an atmosphere of gently moving aircontaining an equivalent quantity of selenium from an upstream source.

As mentioned above, the selenium may be provided as the metal or in anoxidized form such as selenium dioxide, selenious acid, selenic acid andother oxyacids and oxides of selenium. The selenium may also be providedas compounds of selenium, particularly with zinc, such as the selenide,selenite and selenate, oxidizable to and/or decomposable to an oxidizedform of selenium under process conditions. It is noted that the seleniumcontents of the bicharge zinc oxide products obtained are generally -30percent less than the amounts of selenium initially provided.

The method of the invention is further illustrated by means of thefollowing non-limiting examples: I. In static air:

A. A 30 g. sample of conventional photoconductive zinc oxide (Frenchprocess zincoxide prepared by the air combustion of purified zincvapor), average particle size of 0.4u, admixed with 90 mg. of seleniumpowder, was heatedin an open, pyrex glass tube'at 425C for about 20minutes.

by visual inspection.

B. A g. sample of zinc oxide, admixed with 342 mg. of selenous acid washeated in static air at 450C for 20 minutes.

C. A 70 g. sample of zinc oxide, admixed with 292 mg. selenium dioxidewas heated in static air at 450C for 20 minutes.

ll. ln flowing air:

Selenium was vaporized from an upstream source heated at 400C in agentle stream of air. The air stream was passed through a bed of zincoxide at 400C.

The white product from I, (A), above, after cooling in air, wasformulated into a conventional coating mixture (sensitizers, binder andsolvent) of the following composition:

Zinc Oxide 50 grams Thermoplastic Airdrying Resin 14 grams (50% solids)Xylol 35 grams 0.25% Uranine Dye Solution 0.625 gram 0.25% BromophcnolBlue Dye Solution 0.625 gram The above mixture was coated onto aconventional, partially conductive, photocopy base paper to a driedthickness of about 0.5 mil, which corresponds to a 20 lb. coating (i.e.,20 lbs. of dried coating per 3,000 sq. ft.).

The coated sheets, at 3540 percent relative humidity, were charged inthe dark in a' corona current at 40 microamperes for both and corona.Subsequent exposure to light was for 8 seconds at a light intensity of20 ft. candles. The negative photoimage was developed by applying apositive toner; and the positive image was developed by application of anegative or reversal toner. The quality of the photoprints was gradedThe photoproperties of the coated papers, (charge acceptance, dark decayrate, and rate of charge decay in light) were measured and recordedelectronically.

Table lillustrates the effects of initial selenium con-- tentand=processing temperature on image quality of thepositive-chargephotocopy. Image quality is scaled -as follows:

0 non-legible l poor 2 fair 3 acceptable 4 good The photoprints weredeveloped from positive charged'papers coated with dye-sensitized,seleniumtreated photoconductive zinc oxide prepared as described above.The coated papers were given a negative charge before positive charging.

TABLE I IMAGE QUALITY wt Se 400C 450C 500C 0 o 0 0 0.1 l 1 0.2 3 a 0.2 40.3 4 4 3 0.5 3 4 2 1.0 2

Flowing air treatment; others static air.

It is evident that the optimum initial selenium content processing at400450C. Image quality of the print deteriorates markedly when themltlal zmc oxide pro- TABLE m cessmg IS performed below 300 C or above600C.

An ideal photoconductive pigment for use m coated Processing posmvephotocopy papers should accept a charge which would Wt. Temp. in Chargeto Charge Charge remain essentially unchanged in the dark during the pe-C Paper Volts Quality rlod between the end of the charging cycle and thepoint of illumination. A high rate of dark charge decay gives rise to arelatively low charge retention at the 0 400 'h'en :33 0 time ofillumination and, consequently a low contast,

sso low quality photoprmt. I M 400 then 535 3 in Table II, the darkdecay rate of positive charged papers coated with dye-sensitized,selenium-treated 0 450 then 505 0 photoconductlve zmc oxide are shown asa function of their initial selenium contents and the treating condio 250 2 3 tions utilized. The coated papers were prepared as de- 4 scribedabove and given a negative charge before posi- 475 0 s00 then 465 0 tivecharging.

Comparison of the data in Table l with that presented 5 0 in Table llshows that the improvement in positive 500 T 3 charge print qualityobtained is correlated to the resul- 350 tant decrease in the rate ofdark decay (i.e., charge 0 :22??? then 0 h E decay after coronacharging).

TABLE IV Processing Charge Wt. 7: Atmosphere to I Charge PhotoprintSelenium at 400C Post-Treatment Color Coated Acceptance lmage I PaperVolts Quality 580 3 0.2 Static Air None White then 580 3 0.2 Static AirExposed to Air At 550 3 Room Temp. 5 days White then 575 3 50 0 0.2Nitrogen None Buff then 40 0 0.2 Nitrogen Heated in Air At 90 0 200C formin. Buff then 0 0.2 Nitrogen Exposed to Air At 8O 0 Room Temp. 5 daysBuff then O 430 1 0 Static Air None White then 300 0 445 1 0.2 None MildGrey then 210 0 (Added to previous product) I Flowing air treatment;others static sir.

Further experiments illustrating the effects of selenium treatment andprocessing conditions on charge acceptance and image quality aretabulated below:

Table III summarizes the effect of selenium treatment on chargeacceptance. In each instance of selenium treatment, negative andpositive charge acceptance levels were improved. Untreated zinc oxidecontrols the nitrogen-processed pigment products were exposed to air atroom temperature for several days, or heated in air at 200C, the chargeacceptance values were only slightly improved; formulated papers usingsuch pigments still did not provide legible negative or positivephotoprints. These findings appear contrary to the disclosures ofJapanese Pat. publication Nos. 16,427/66 and 16,428/66, which teachselenium treatment in an inert or reducing atmosphere to effectimprovements in zinc oxide photo-sensitivity.

The last two examples in Table IV show that heated, but seleniumuntreated, conventional photoconductive zinc oxide gave fair negativephotoprints, but no legible positive photoprints. The addition ofpowdered selenium after heat treatment had no beneficial action.

The white products from [(B) and l(C) above were individually formulatedas coating mixtures and applied to photocopy base paper as describedherein. Both products gave good quality positive charge photocopy; theirelectric properties were also good, with dark decay rates of 5 and 7volts/second, respectively. Comparable results were obtained usingproducts wherein zinc oxide was treated with selenium in a flowing airsystem as in II above.

Selenium treatment of photoconductive zinc oxide according to the methodof the present invention provided improved, good quality, high contrastphotoprints when exposed to a negative charge and then developed into anegative photoprint. For positive photocopying, the eoated paper wasgiven a negative charge followed promptly by a positive charge, afterwhich the positive print was developed. The use of positive chargingwithout a prior negative charging also provided positive photoprints butimage quality is slightly less.

lclaim:

l. A method for producing bicharge photoconductive zinc oxide whichcomprises mixing photoconductive zinc oxide with 0.022.0 weight percentof selenium in the form of elemental selenium or as an oxide or oxyacidof selenium or a compound of selenium oxidizable or decomposable to anoxidized form of selenium under the process conditions and heating themixture in air at 300600C.

2. A method according to claim 1 wherein heating is effected at 400-500Cin static air for 10-30 minutes.

3. A method for producing bicharge photoconductive zinc oxide whichcomprises mixing photoconductive zinc oxide with 0.1-1.0 weight percentof selenium in the form of elemental selenium or as an oxide or oxyacidof selenium or a compound of selenium oxidizable or decomposable to anoxidized form of selenium under the process conditions and heating themixture in air at 400500C.

4. A method for producing bicharge photoconductive zinc oxide whichcomprises heating selenium in a flowing air stream at 400-500C andpassing the air stream through a bed of photoconductive zinc oxide at400500C, the air passed through the bed containing 0.022.0 weightpercent of selenium based on the zinc oxide.

5. Colorless bicharge photoconductive zinc oxide produced by heatingphotoconductive zinc oxide in air at 300600C in the presence of 002-20weight percent of selenium based on the zinc oxide in the form ofelemental selenium or as an oxide or oxyacid of selenium or a compoundof selenium oxidizable or decomposable to an oxidized form of seleniumunder the process conditions.

6. Bicharge photoconductive zinc oxide suitable for negative andpositive photocopy produced by heating photoconductive zinc oxide in airat 300600C in the presence of 002-20 weight percent of selenium based onthe zinc oxide in the form of elemental selenium or as an oxide oroxyacid of selenium or a compound of selenium oxidizable or decomposableto an oxidized form of selenium under the process conditions.

2. A method according to claim 1 wherein heating is effected at400*-500*C in static air for 10-30 minutes.
 3. A method for producingbicharge photoconductive zinc oxide which comprises mixingphotoconductive zinc oxide with 0.1-1.0 weight percent of selenium inthe form of elemental selenium or as an oxide or oxyacid of selenium ora compound of selenium oxidizable or decomposable to an oxidized form ofselenium under the process conditions and heating the mixture in air at400*-500*C.
 4. A method for producing bicharge photoconductive zincoxide which comprises heating selenium in a flowing air stream at400*-500*C and passing the air stream through a bed of photoconductivezinc oxide at 400*-500*C, the air passed through the bed containing0.02-2.0 weight percent of selenium based on the zinc oxide. 5.Colorless bicharge photoconductive zinc oxide produced by heatingphotoconductive zinc oxide in air at 300*-600*C in the presence of0.02-2.0 weight percent of selenium based on the zinc oxide in the formof elemental selenium or as an oxide or oxyacid of selenium or acompound of selenium oxidizable or decomposable to an oxidized form ofselenium under the process conditions.
 6. Bicharge photoconductive zincoxide suitable for negative and positive photocopy produced by heatingphotoconductive zinc oxide in air at 300*-600*C in the presence of0.02-2.0 weight percent of selenium based on the zinc oxide in the formof elemental selenium or as an oxide or oxyacid of selenium or acompound of selenium oxidizable or decomposable to an oxidized form ofselenium under the process conditions.